This invention relates to apparatus for the spatial adjustment and holding of individual magnet coils which are lined up one behind the other on a common axis and serve to generate a homogeneous magnetic base field of a facility for nuclear spin tomography, the positions and orientations of the magnet coils being adjustable by means of mechanical adjusting devices which are connected to stationary parts of a frame structure. Such an adjusting and holding apparatus is known, for instance, from U.S. Pat. No. 4,315,216.
In the field of medical diagnostics, image-displaying methods have been developed, in which an image similar to an x-ray tomogram is constructed by numerical analysis or measurement of integral resonance signals of nuclei such as protons from the spatial spin density and/or relaxation time distribution of a body to be examined. This method is also called zeugmatography ("Nature", Vol. 242, 1973, pages 190 and 191).
Since, as is well known, the nuclear resonance signal increases with increasing magnetic base field, a base field which is as strong as possible and which has sufficient homogeneity in a measuring range is desired for nuclear spin tomography apparatus. On this base field, stationary and/or pulsed gradient fields are superimposed. In addition, a high-frequency field oriented perpendicularly to the base field must be provided. The dimensions of the respective magnet coils must be matched to the dimensions of the body to be examined in such a way that the latter can be inserted without difficulty into the measuring region surrounded by the coils.
U.S. Pat. No. 4,315,216 discloses a magnet system for generating a base field, which system comprises four ring-shaped magnet coils which are aligned along a common horizontal axis. Since, as is well known, the homogeneity of the field of these coils must meet stringent requirements and should not vary, for instance, more than 0.01% of the magnetic field strength in the entire measuring volume, the positions and orientations of individual coils of the magnet system must be finely adjustable. Such adjustability is necessary even in cases of the highest manufacturing precision with respect to the magnet coils in order to compensate, for instance, the influence of iron parts in the vicinity of the magnet system. Each of the four magnet coils has maximally three degrees of freedom of translation and two degrees of freedom of rotation. Accordingly, a number of devices must be provided for mutually aligning the individual magnet coils. In this magnet system, all four adjustable magnet coils are supported on a common base plate. At one end face of each of the magnet coils are fastened four circumferentially equispaced plate-shaped elements which project radially outwardly. Corresponding plate-shaped elements of the four magnet coils are connected to each other via a common threaded rod. The axial positions and angular orientations of the individual coils within the frame structure are precisely determined by adjusting fastening nuts, by which plate-shaped elements of the coils are secured to the respective threaded rods. Furthermore, the individual magnet coils of the known magnet system can be aligned in the vertical direction via adjusting devices engaging the base plate (see FIGS. 4a and 4b of the above-identified patent). In this known magnet system, the expenditure for adjustment is relatively large since five adjusting devices are provided for each magnet coil. Furthermore, because the individual magnet coils are connected to each other via the threaded rods, thermal length changes and vibrations of these coils lead to a de-adjustment of the entire system and thus to a corresponding degradation of the homogeneity of the magnetic base field. Readjustment is relatively difficult in this case.
It is an object of this invention to provide an improved adjusting and holding apparatus of the above-described type, in which a relatively simple adjustment of the individual magnet coils is possible and, at the same time, a high homogeneity of the magnetic base field can be achieved.